The present invention relates to a control system for a variable geometry turbocharger.
Turbochargers comprise a turbine driven by exhaust gas delivered to an exhaust gas inlet of the turbocharger from an engine exhaust manifold. The turbine drives a compressor which delivers air to an engine intake manifold via an air outlet of the turbocharger. In a variable geometry turbocharger, physically displaceable components are located in the exhaust gas inlet of the turbocharger so that the geometry of the inlet can be adjusted to control the exhaust gas pressure upstream of the turbine and the speed at which the exhaust gas flows through the turbine. This in turn affects the speed of rotation of the turbine, and hence the speed of rotation of the associated compressor. Generally variable geometry mechanisms in turbochargers are displaced in order to appropriately modulate the intake manifold pressure of the engine to optimize engine running conditions.
Electronic variable geometry turbocharger control devices are known which receive input signals representing different engine operation parameters and generate a control signal which is used to control an actuator which in turn controls the geometry of the variable mechanism so as to achieve desired running conditions. Most of the known control devices are designed to maintain a desired engine intake manifold pressure or density as a function of engine speed and fuel rate. The control signal may also be modified to take into account for example ambient temperature and pressure, charge air temperature, turbocharger rotational speed and throttle demand.
The known devices may define closed loop control systems in which a control signal is generated by comparing a direct measurement of the intake manifold pressure with a desired value of this parameter, or by comparing a calculated actual inlet manifold density with a desired value of this parameter. The magnitude of the difference revealed by the comparison is used to derive a control signal which determines the displacement of the variable geometry mechanism. Such devices are satisfactory for steady or quasi-steady state operation, but do not operate well in rapidly changing transient conditions. For example, if there is a sudden increase in the demanded intake manifold pressure, perhaps as a result of a large change in power demand, the natural action of the control device is to drive the control signal in such a way as to reduce the turbine inlet area in order to increase intake manifold pressure and density. If the system has been calibrated for fast response, the variable geometry mechanism will probably reduce the turbine area to a minimum value very rapidly in order to increase the intake manifold pressure and density as rapidly as possible, before increasing the turbine area again to some new quasi-steady state area as the new desired intake manifold pressure and density is approached. Unfortunately, as the turbine area is reduced during the initial phase of a transient change in conditions, the exhaust manifold pressure rises much more rapidly than the intake manifold pressure as a result of the different time constants of the exhaust and intake systems. If this effect is not properly regulated, the end result is a large negative pressure differential across the engine, that is between the engine intake manifold and the engine exhaust manifold, and this causes a large reduction in engine volumetric efficiency. This results in the mass flow of air into the engine reducing in spite of the increased inlet manifold pressure. Thus, even though the rate of rise of the intake manifold pressure may have been very rapid, the ability of the engine to accept load will have been reduced. To the driver of the engine this is detected as a loss of performance and an increase in smoke emissions. In addition, transient very high exhaust manifold pressures may be generated, which can adversely affect the engine.
The above problem of an unduly rapid exhaust manifold pressure rise is avoided in the known control systems by limiting the rate of rise of the exhaust manifold pressure. This may be achieved by limiting the response rate of the control device, or by limiting the minimum turbine area to a relatively large value; for example, by imposing a physical limitation within the turbine or by imposing limit values on the control signal generated by the control system. These solutions to the problem compromise the transient performance of the engine and turbocharger. They are also susceptible to large errors because they are open loop systems in which no information related to the actual engine volumetric efficiency is fed back into the control loop. In addition, extra margins have to be added to allow for production variations, ambient pressure and temperature variations, and lifetime variations in the performance of the engine and turbocharger. All of these factors compromise performance.
It is also known for variable geometry turbochargers to modulate the engine braking power by acting on the exhaust flow; for example, by restricting the exhaust flow. The known engine braking control systems monitor several input signals. representative of engine operating parameters and generate a control signal which is applied to an actuator of the variable geometry mechanism so as to achieve the desired braking power. The known systems are open loop, however, as no direct measurement of braking power is fed back to the controller. It is possible to cause damage to the engine if the variable geometry mechanism restricts the turbine inlet area too much so that an excessive exhaust manifold pressure is generated. Therefore current braking control devices using open loop control are always designed with wide safety margins to account for production variations, ambient pressure and temperature variations, and lifetime variations in the performance of the engine and turbocharger. The result of all these constraints is that engine braking power is significantly reduced as compared with what is potentially available.
It is an object of the present invention to provide a variable geometry turbocharger control system which obviates or mitigates the problems outlined above.
According to the present invention, there is provided a control system for a variable geometry turbocharger having a turbine driven by exhaust gas delivered to an exhaust gas inlet of the turbocharger from an engine exhaust manifold and a compressor driven by the turbine to deliver air to an engine intake manifold via an air outlet of the turbocharger, comprising means for monitoring a parameter which is a function of the pressure within the engine exhaust manifold, and closed loop control means for controlling the displacement of a variable geometry mechanism located upstream of the turbine to maintain the monitored parameter within predetermined limits.
The monitored parameter may be a function of the density of the gas within the exhaust gas manifold, or of the pressure within the engine exhaust manifold, but is preferably related to the difference between the pressures within the engine exhaust manifold and the engine intake manifold. For example, the monitored parameter may correspond to the difference between the pressures within the engine exhaust manifold and the engine intake manifold divided by either the pressure within the engine exhaust manifold or the pressure within the engine intake manifold.
The predetermined limit may be fixed, or may be derived from the engine operation conditions; for example, engine speed and fuel rate.
A switch may be provided to select either a first control signal intended to control the variable geometry mechanism to maintain a desired engine intake manifold pressure or density, or a second control signal intended to control a variable geometry mechanism to maintain the monitored parameter within the predetermined limits. The switch may be controlled in dependence upon whether or not the second signal exceeds the predetermined limits. The selection of either the first or second control signals may be overridden by a third control signal intended to reduce turbocharger speed if the turbocharger speed exceeds a predetermined limit, or by a fourth control signal intended to control the variable geometry mechanism to provide a desired exhaust manifold pressure during engine braking when the engine is in braking mode.
The invention provides closed loop control of engine volumetric efficiency which enables the optimization of engine performance through transient operating conditions and an appropriate response to transient conditions. In addition, when the system is operating in braking mode, the exhaust pressure can be used as a feedback signal to achieve optimized braking performance. Adverse effects on the engine can be avoided under all conditions. Thus the transient response of the engine and the engine braking power may be maximized without exceeding safety limits. The system also compensates for production variations, ambient pressure and temperature variations, and lifetime variations in the performance of the engine and turbocharger.